Re: fs: locks: WARNING: CPU: 16 PID: 4296 at fs/locks.c:236 locks_free_lock_context+0x10d/0x240()

[Jeff Layton]

On Wed, 14 Jan 2015 18:44:41 -0500
Sasha Levin <sasha.levin@xxxxxxxxxx> wrote:

> On 01/14/2015 09:27 AM, Jeff Layton wrote:
> > On Tue, 13 Jan 2015 17:50:45 -0500
> > Sasha Levin <sasha.levin@xxxxxxxxxx> wrote:
> > 
> >> On 01/13/2015 04:44 PM, Jeff Layton wrote:
> >>> On Tue, 13 Jan 2015 00:11:37 -0500
> >>> Sasha Levin <sasha.levin@xxxxxxxxxx> wrote:
> >>>
> >>>> Hey Jeff,
> >>>>
> >>>> While fuzzing with trinity inside a KVM tools guest running the latest -next
> >>>> kernel, I've stumbled on the following spew:
> >>>>
> >>>> [  887.078606] WARNING: CPU: 16 PID: 4296 at fs/locks.c:236 locks_free_lock_context+0x10d/0x240()
> >>>> [  887.079703] Modules linked in:
> >>>> [  887.080288] CPU: 16 PID: 4296 Comm: trinity-c273 Not tainted 3.19.0-rc4-next-20150112-sasha-00053-g23c147e02e-dirty #1710
> >>>> [  887.082229]  0000000000000000 0000000000000000 0000000000000000 ffff8804c9f4f8e8
> >>>> [  887.083773]  ffffffff9154e0a6 0000000000000000 ffff8804cad98000 ffff8804c9f4f938
> >>>> [  887.085280]  ffffffff8140a4d0 0000000000000001 ffffffff81bf0d2d ffff8804c9f4f988
> >>>> [  887.086792] Call Trace:
> >>>> [  887.087320] dump_stack (lib/dump_stack.c:52)
> >>>> [  887.088247] warn_slowpath_common (kernel/panic.c:447)
> >>>> [  887.089342] ? locks_free_lock_context (fs/locks.c:236 (discriminator 3))
> >>>> [  887.090514] warn_slowpath_null (kernel/panic.c:481)
> >>>> [  887.091629] locks_free_lock_context (fs/locks.c:236 (discriminator 3))
> >>>> [  887.092782] __destroy_inode (fs/inode.c:243)
> >>>> [  887.093817] destroy_inode (fs/inode.c:268)
> >>>> [  887.094833] evict (fs/inode.c:574)
> >>>> [  887.095808] iput (fs/inode.c:1503)
> >>>> [  887.096687] __dentry_kill (fs/dcache.c:323 fs/dcache.c:508)
> >>>> [  887.097683] ? _raw_spin_trylock (kernel/locking/spinlock.c:136)
> >>>> [  887.098733] ? dput (fs/dcache.c:545 fs/dcache.c:648)
> >>>> [  887.099672] dput (fs/dcache.c:649)
> >>>> [  887.100552] __fput (fs/file_table.c:227)
> >>>
> >>> So, looking at this a bit more...
> >>>
> >>> It's clear that we're at the dput in __fput at this point. Much earlier
> >>> in __fput, we call locks_remove_file to remove all of the locks that
> >>> are associated with the file description.
> >>>
> >>> Evidently though, something didn't go right there. The two most likely
> >>> scenarios to my mind are:
> >>>
> >>> A) a lock raced onto the list somehow after that point. That seems
> >>> unlikely since presumably the fcheck should have failed at that point.
> >>>
> >>> ...or...
> >>>
> >>> B) the CPU that called locks_remove_file mistakenly thought that
> >>> inode->i_flctx was NULL when it really wasn't (stale cache, perhaps?).
> >>> That would make it skip trying to remove any flock locks.
> >>>
> >>> B seems more likely to me, and if it's the case then that would seem to
> >>> imply that we need some memory barriers (or maybe some ACCESS_ONCE
> >>> calls) in these codepaths. I'll have to sit down and work through it to
> >>> see what makes the most sense.
> >>>
> >>> If your debugging seems to jive with this, then one thing that might be
> >>> interesting would be to comment out these two lines in
> >>> locks_remove_flock:
> >>>
> >>>         if (!file_inode(filp)->i_flctx)
> >>>                 return;
> >>>
> >>> ...and see if it's still reproducible. That's obviously not a real fix
> >>> for this problem, but it might help prove whether the above suspicion
> >>> is correct.
> >>
> >> Removing those two lines makes the issue go away.
> >>
> >> I'm guessing that figuring out which filesystem we were abusing isn't
> >> interesting anymore...
> >>
> > 
> > Sigh. I've been trying to reproduce this today. I've set up two
> > different KVM guests on two different hosts, and run trinity on both,
> > and I can't seem to get this warning to pop.
> > 
> > Could you share what trinity command-line options you're using? Any
> > other special setup I should be considering to reproduce it?
> > 
> > I was hoping to get it to reproduce so I could test out potential
> > memory barrier fixes...
> 
> I don't think there's anything special about my setup here that can
> trigger that, specially if it's not dependant on a filesystem in use.
> 
> I'm running trinity with: ./trinity -xsched_setattr -xsetpriority
> -xunshare -xreboot -xshutdown -xnfsservctl -xclock_nanosleep -xuselib
> -xperf_event_open -m --quiet --dangerous -C 400 -l off
> 
> If that doesn't end up helping, I'd be happy to test out fixes here,
> it usually reproduces quickly.
> 
> 

Great, I'll try those options tomorrow.

I did push out a newer version of the series to linux-next, which does
a smp_rmb at the beginning of locks_remove_file. I'm not sure if that's
sufficient to fix this, but it's probably a reasonable start.

If you update the kernel you're testing to the next linux-next tree,
then it'd be interesting to know if it's still reproducible.

Thanks again!
-- 
Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
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